Field lighting network with a distributed control system

ABSTRACT

A field lighting network providing for individual control of the light fittings while reducing overall cable costs. A converter unit converts a supply voltage obtained from an A.C. main to a substantially constant current in a Boucherot circuit with a series resonance circuit, tuned to the main frequency. The converter unit includes a Boucherot circuit having a series resonance circuit, substantially tuned on the main frequency, and an additional inductance in series with a load connected to the converter unit. A regulator unit supplied with current couples to each fitting or group of fittings for individual regulation of the current passing through the respective lamp or lamps. Each regulator unit is disposed to receive control information on the power cable.

BACKGROUND OF THE INVENTION

The present invention relates to a field lighting network including aplurality of series-connected light fittings, supplied from an A.C. mainvia a converter unit adapted to convert the substantially constantvoltage obtained from the main to a substantially constant current inoutgoing current lines containing the fittings.

A network of this kind is described in U.S. Pat. No. 4,754,201.

The traditional method of controlling and monitoring field lights on anairfield is to supply power to the different light configurations via aso-called parallel system or a so-called series system (FIGS. 1 and 2).In such a case, the regulating and monitoring unit is centrally placedin a cabinet or the like, and its regulators provide either a constantvoltage (parallel system) or a constant current (series system) to thedifferent power supply cables of the different field lightconfigurations.

The object of the present invention is to provide a field lightingnetwork of the kind described above, wherein individual control of thelight fittings, or groups thereof, is possible while cable costs areconsiderably reduced at the same time.

SUMMARY OF THE INVENTION

In the field lighting network according to the present inventiondifferent light configurations are supplied by one or more transformers,implemented in such a way that they may be regarded as representingcurrent supply sources. Each light fitting is provided with a localregulating and monitoring unit, which obtains its control informationvia signals carried by the power cable, a separate control cable or byradio. In the field lighting network of the present invention there isthus used a "current supply" network where the prevailing output voltageis a function of the prevailing load.

The advantages accompanying the use of such a current supply system in afield lighting network for airfields are as follows:

1) The lamps have a resistance that varies greatly, depending on thefilament temperature. A current supplying system provides a smoothsuccessive voltage increase across the lamp, whereas a voltage supplyingsystem results in severe current surges when the lamp is turned on.

2) The lamps are spread over large areas, and if a current supplyingsystem is used, single conductor, high-voltage cables, typically for 5kW, can be used for the supply, which considerably reduces cable costs.

3) Current transformers are cheaper than corresponding voltagetransformers.

In a preferred embodiment of the network of the present invention theconverter unit adapted for converting the voltage obtained from the A.C.main to a substantially constant current is a Boucherot circuit with aseries resonance circuit, tuned to the main frequency. This is a simpleand advantageous method of obtaining a current source having anindefinite EMF behind an infinite impedance. The Boucherot circuit isdescribed more in detail by E. Arnold, Die Wechselstromtechnik, ErsterBand, Zweite Auflage, Verlag Julius Springer, Berlin, pp 141-4.

According to another embodiment of the network of the present inventionthe converter unit includes a further inductance in series with a loadconnected to the converter unit. If this inductance is of the samemagnitude as the one included in the series resonance circuit, duringidling (i.e. short-circuiting of the current system), the current in thenetwork ideally will be zero.

Another advantage in the utilization of this special Boucherot circuitis that the effect on the network is small and that the sinus wave shaperemains essentially unaffected, which facilitates signal transmissionover the power cables. The Boucherot circuit is generally advantageousin applications for airfields, where a low interference level isessential.

In accordance with a further embodiment of the network according to theinvention, the regulating unit includes a counter synchronized with thecurrent zero crossings and provided with its own oscillator controlledby a binary number. This binary number can be varied individually foreach lamp, and is determined, preferably, from a central control system.

In accordance with a still further embodiment of the present inventionthe regulating unit includes a triac connected in parallel with thelight fitting lamp, for regulating the current through the lamp bycontrolling the ignition time.

The network, in accordance with the invention, also preferably includesa safety system having three levels, since a fault that could lead to anopen circuit would cause impermissibly high voltages. The networkaccording to the invention therefore includes transient protection,primarily in the shape of a component (e.g. a type of two-way Zenerdiode), which is connected across each lamp and which is short-circuited(not interrupted) when it is driven outside its operating range. Asfurther protection, the triac can be disposed such that in response toovervoltage occurring across the lamp it is forced to a permanent "on"state for short-circuiting the transients, and, as a third protectionmeans, there can be arranged a (mechanical and/or electronic) device forshort-circuiting any occurring overvoltages, if these are notshort-circuited by the other protective means.

In order to explain the invention in more detail, an embodiment of thenetwork according to the invention, selected as an example, will bedescribed with reference to the diagrams below.

FIGS. 1 and 2 illustrate the principles of parallel and series supply,respectively, for field lightings on an airfield, according to priorart;

FIG. 3 illustrates the principle of the network according to the presentinvention;

FIG. 4a illustrates the basic implementation of a Boucherot circuitincluded in the converter unit of the network according to the presentinvention;

FIG. 4b illustrates the electrical properties of the circuit;

FIG. 5 illustrates a further refinement of the Boucherot circuit;

FIG. 6 illustrates the further developed Boucherot circuit of FIG. 5included in the network according to the invention;

FIG. 7 schematically illustrates an example of a local regulating andmonitoring unit in the network according to the invention; and

FIG. 8 illustrates the unit of FIG. 7 in more detail.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In FIG. 3 there is schematically illustrated an embodiment of thenetwork according to the invention, in which a series system of aplurality of light fittings is supplied from a current generator 10.Each fitting includes a lamp 6 as well as a local regulating andmonitoring unit 12. The output voltage is not regulated, and becomes afunction of the prevailing load. The regulating and monitoring units 12are given their control information, from a central control system, bysignals carried on the power cable, a separate control cable or byradio.

The current source is realized by a converter unit supplied from an A.C.main having substantially constant voltage. This converter unit convertsthe voltage obtained from the main to a substantially constant currentin the outgoing lines that include the light fittings.

The converter unit includes a Boucherot circuit, illustrated in itsbasic implementation in FIG. 4a. The circuit contains a series resonancecircuit formed of an inductance L_(N) and a capacitor C and is tunedsubstantially to the main frequency.

The properties of the Boucherot circuit are as follows. When it issupplied with the voltage U_(N) from the main the voltage seen from theload side is infinitely great when the load impedance goes towardsinfinity, and for a short-circuited load, the impedance is formed of thereactance in the inductance L_(N) (FIG. 4b).

Applying Thevenin's theorem, the circuit may be represented by aninfinitely great EMF behind an infinite impedance (i.e. it constitutes acurrent source). The magnitude of the current is: I=U_(N) /X, where X=ωL_(N) is the reactance of the inductance, and this current is equal tothe short-circuiting current. In case of a short-circuit, the current inthe load line L_(N) =I and is purely inductive.

In FIG. 5 there is shown a further refinement of the Boucherot circuit,which is used in the network according to the present invention. In thisembodiment a second inductance L₂ is connected in series with the loadZ_(bel). If the inductance L₂ is of the same magnitude as the seriesresonance circuit inductance L_(N), one of the advantages of thisembodiment is that the main current L_(N) is equal to zero when thesystem is short-circuited, i.e., in a no-load state, since L₂ and C arein parallel resonance.

In the description thus far of the Boucherot circuit the load has beenassumed to be linear, namely a resistance in series with an (ideal)inductance. In the network according to the invention, the load consistsof a resistance, i.e. the lamp 6, which is connected in parallel with atriac 8 (FIGS. 6-8). The effective value of the current through the lampcan then be varied by varying the ignition angle of the triac 8. Thiscombined load is non-linear, but in spite of this the current from theBoucherot circuit is practically sinusoidal, due to the inductance L₂ atthe output. As previously mentioned, this affords important advantages.

When the triac 8 is disconnected at the beginning of each half periodthe Boucherot circuit is resistively loaded, and when the triac 8 isconnected for the rest of the half period the Boucherot circuit isshort-circuited. The wave form of the voltage across the load is alsoformed of a portion of a sinus form that can be divided into fundamentaltone and overtones. The overtones will be (almost) filtered away by theinductance and capacitance of the circuit. The fundamental tone of thevoltage can be divided into an active component in phase with thecurrent, and a reactive component phase shifted 90° forward of thecurrent. In other words, the load acts as a resistive-inductive load.

In FIG. 6 there is shown an example of a series system of field lightsof the kind to which the invention relates, and supplied from aBoucherot circuit via a current transformer 14 on the output side. Theseries line is loaded by a plurality of current transformers 2, each ofwhich is connected to one or more light fittings on the secondary side.Via a switch 16 the Boucherot circuit is connected between the phases ofan ordinary 3-phase main 18. Several such circuits can be connecteddistributed between the phases of the main to balance the 3-phase load.

As already mentioned, the network must be provided with protectivemeans, since very high voltages will occur if a light fitting shouldform an open circuit, e.g., because of a lamp failure.

The triac 8 connected in parallel with the lamp 6 is adapted to bepermanently turned on for short-circuiting the lamp, should the lampfail. If the circuit for turning on the triac does not function, thereis a second overvoltage protection in the form of a two-way Zener diode20 connected across the lamp 6, and it will be short-circuited if anovervoltage occurs across the lamp. The Boucherot circuit is furtherprotected by a short-circuiting means comprising two anti-parallelconnected thyristors 22 across the output transformer 14. If the linewith the transformers should form an open circuit, e.g., due to a lampfailure, and the voltage across the transformer 14 rises, theshort-circuiting means 22 will start to function and short-circuit theBoucherot circuit. If the operation mechanism of the short-circuitingmeans 22 fails, a break-down will occur in the thyristor as a result ofthe overvoltage, and a permanent short-circuit will be established. Onlya limited overvoltage will appear in the network for a very short time,and this overvoltage can be used to activate an alarm and to trigger theswitch 16, with a delay of a few periods, so that the current has timeto dissipate.

The network shown in FIG. 6 thus includes a threefold overvoltageprotection.

As mentioned above in connection with the description of FIG. 3, eachlight fitting includes a local regulator unit 12 (not shown in FIG. 6).An example of such a unit is illustrated in FIG. 7.

The regulating and monitoring unit includes a conventional currenttransformer 2, connected between the power supply 4 and the lamp 6, aswell as a triac 8 connected in parallel with the lamp 6, for regulatingthe light intensity of the latter. Thyristors can be used instead of thetriac 8 for regulating illumination. The current transformer 2 drives aconstant current through the secondary side, and with the triac 8 turnedoff the entire secondary side current flows through the lamp 6. Bygradually turning on the triac 8 a gradually decreasing current flowsthrough the lamp 6. The light intensity from the lamp can thus beregulated in the method explained in greater detail in connection withFIG. 8.

The regulating and monitoring unit illustrated in FIGS. 7 and 8 may beessentially divided into four parts: Power supply, detector, counter andamplifier.

The power supply includes an auxiliary transformer 24, which may be acurrent transformer having a high transformation ratio, the secondaryside of which is connected to a rectifier bridge 26. The rectifiedoutput voltage from the rectifier bridge 26 is smoothed by a capacitor28 and stabilized by a Zener diode 30.

The detector is connected to the A.C. terminals of the rectifier bridge26, where the voltage has a square wave configuration and is in phasewith the current in the line containing the light fittings. Thesteepness of the flanks of the square wave are improved with the aid ofcomparators 32, 34 and the square wave is converted into a short pulsePE, which is repeated every half period by transferring the outputvoltages of the comparators 32, 34 to the base of a transistor 36 viatheir respective capacitors 38, 40. This zero point detector will thussend a pulse PE for each zero crossing of the current in the linecontaining the light fittings.

The counter includes a crystal-controlled oscillator with a binarycounter 42, which generates a clock pulse C1, which in turn clocks afollowing 8 bit binary count-down counter 44. The count-down counter 44is activated by the pulse PE which sets it to the binary number N, to befound at the inputs JO, J1 . . . J7. After N counts, the count-downcounter 44 delivers a short output pulse CO. This pulse CO sets an RSflip-flop to zero 46, which is set to the "one" state by the pulse PE.The pulse CO sets the output of the flip-flop 46 to 0, in which state itremains for the rest of the half period. The output signal P isamplified in the amplifier 48 and forms the control pulse turning on thetriac 8, which is turned on for P=O. The pulse trains PE, CO and P areshown in the upper right-hand part of FIG. 8.

The binary number N is individual for each lamp 6 and is transferred tothe address of the light fitting in question from a computer in thecentral control system. This transfer is most economically achieved byusing the power cable, but it can also be effected via separate signalcables or by radio, as already mentioned.

As mentioned earlier, there is a means for switching the triac to apermanent on-state in case of a lamp failure, and there are also means(not shown) for sensing the condition of the lamp 6 and sending thatinformation back to the central control system computer, which can thuskeep count of which lamps need to be changed.

We claim:
 1. A field lighting network, including a plurality ofseries-connected light fittings supplied from an A.C. main via aconverter unit, adapted to convert a substantially constant voltageobtained from the main to a substantially constant current in departingcurrent lines containing the light fittings, the network comprising:aregulator unit (12) supplied with current being associated with eachfitting or group of fittings for individual regulation of the currentpassing through the associated lamp or lamps (6), wherein each regulatorunit (12) is disposed to receive control information on the power cable,and wherein the converter unit includes a Boucherot circuit having aseries resonance circuit (L_(N) C), substantially tuned on the mainfrequency, and an additional inductance (L₂) in series with a load(Z_(bel)) connected to the converter unit, said inductance beingpreferably of equal magnitude as the inductance included in the seriesresonance circuit.
 2. The network as claimed in claim 1, wherein theregulator unit (12) includes a counter (42, 44) synchronized to the zerocrossings of the current, said counter being intended for currentregulation controlled by a set binary number.
 3. The network as claimedin claim 1, wherein the regulator unit includes a triac (8) or thyristorconnected in parallel with the lamp (6) of the light fitting forregulating the current through the lamp.
 4. The network as claimed inclaim 1, wherein the regulator unit also includes means for monitoringthe operational state of the lamp (6) in the light fitting.
 5. Thenetwork as claimed in claim 1, further comprising an overvoltageprotection component, preferably a two-way Zener diode (20), which isshort-circuited when it is driven outside its operation range, saidcomponent being connected across each lamp (6).
 6. The network asclaimed in claim 3, wherein the triac (8) connected in parallel with thelamp is adapted to be forced in a permanent on-state in response to theoccurrence of overvoltage across the lamp (6) for short-circuiting untila resetting signal is given.
 7. The network as claimed in claim 6,further comprising a short-circuiting means (22) arranged across theprimary side of a transformer (14) connected to the output of theBoucherot circuit for short-circuiting the transformer if an overvoltagecondition occurs.
 8. The network as claimed in claim 1, wherein theregulator units (12) are adapted for being controlled from a centralcontrol system.